1. Field of the Invention
The present invention relates to the analysis of seismic data, and particularly to a method of first arrival picking of seismic refraction data using the τ-p transform on energy ratio seismic shot records to enhance the determination of first arrival events.
2. Description of the Related Art
In the oil and gas industry, seismic surveys are one of the most important techniques for discovering the presence of subterranean hydrocarbon deposits. If the data is properly processed and interpreted, a seismic survey can provide geologists with a two-dimensional (2-D) or three-dimensional (3-D) representation of subsurface lithologic formations and other features, so that they may better identify those formations likely to contain oil and/or gas. Having an accurate representation of an area's subsurface lithologic formations can increase the odds of hitting an economically recoverable reservoir when drilling, and decrease the odds of wasting money and effort on a nonproductive well.
Oil and gas are often trapped thousands of feet below the earth's surface. To find the oil, geologists and geophysicists typically use either two-dimensional (2D) or three-dimensional (3D) seismic surveys. To perform these seismic surveys, an acoustic wave generated by a shot, e.g., dynamite or a mechanical vibrator, is propagated downward and is refracted back when it encounters a geological discontinuity. This signal is recorded by a geophone as a trace.
To gather high-fold surveys, land seismic survey operations typically require placing hundreds to thousands of geophones at locations about the area to be surveyed. When a seismic source is generated, either as an impulse caused by dynamite or a vibration sweep caused by a mechanical apparatus carried by a truck, the seismic reflections are detected by the geophones. The seismic data generated by all the geophones is then transmitted to a central recording system.
The amount of seismic data transmitted to the central recording system may be considerable. For example, a 20-second vibration sweep can generate on the order of 250,000 bits of data. When there are 1,000 geophone channels in use, this translates to 250,000,000 bits of data every 20 seconds, or an effective data rate of 12.5 megabits per second. Increasing the number of geophone channels increases the amount of seismic data to be transferred to the central recording system. Many current seismic survey projects have more than 10,000 geophones active at any one time, and the requirements for more channels are increasing. In a few years time, it is expected that channel counts as high as 100,000 will not be uncommon. These data rates put tremendous strain on traditionally used seismic data processing techniques.
Once the high-fold surveys are taken, the seismic data processing of all the collected seismic data begins. One of the processing steps is determining the “first arrival” or “first break” for each of the traces. The first arrival indicates a refraction of the acoustic energy upon encountering a geological discontinuity, and the timing of the first arrival, or first arrival time, is important in determining the depth of the refractor and performing corrections to a stack of seismic traces. Historically, a geophysicist would manually pick the first arrival for each trace of a seismic stack. This process was time-consuming, and several auto-picking methods emerged, including those using energy ratios, fractals, and neural networks to automatically determine the first arrivals.
Unfortunately, these auto-picking methods are not particularly adapted for use with 3D surveys. For 3D surveys, interactive first arrival picking is common. Using this method, an interpreter sits at a workstation, displays shot gathers, and uses an auto-picker to select first arrivals. Quality control is achieved by interactive editing in the shot, receiver, and offset domains. This process can take months for large 3D surveys having high channel counts and consisting of millions of traces.
The data in a gather is typically recorded or first assembled in the time-offset domain. That is, the seismic traces recorded in the gather are assembled or displayed together as a function of offset (i.e., the distance of the receiver from a reference point) and of time. The time required for a given signal to reach and be detected by successive receivers is a function of its velocity and the distance traveled. Those functions are referred to as kinematic travel time trajectories. Thus, at least in theory, when the gathered data is displayed in the time-offset domain (the (t-x) domain), the amplitude peaks corresponding to reflection signals detected at the receivers should align into patterns that mirror the kinematic travel time trajectories. It is from those trajectories that one ultimately may determine an estimate of the depths at which formations exist.
To achieve this determination, there exist many methods that seek to achieve a separation of the refracted first-arrival signal from noise by transforming the data from the (t-x) domain to other domains, such as the frequency-wavenumber (F-K) or the time-slowness (tau-P) domains, in order to minimize overlap between the signal and noise data. Once the data is transformed, various mathematical filters are employed to the transformed data to eliminate as much of the noise as possible in an effort to enhance the primary reflection signals. The data is then inverse-transformed back into the (t-x) domain for interpretation or further processing. For example, so-called Radon filters are commonly used to attenuate or remove multiple reflection signals. Such methods rely on Radon transformation equations to transform data from the (t-x) domain to the τ-p domain, where it can be filtered. More specifically, the (t-x) data is transformed along kinematic travel time trajectories having constant velocities and slowness, where slowness p is defined as the reciprocal of velocity (p=1/v). While various mathematical methods have been developed to process seismic data in such a way that noise is separated from primary reflection signals, there remains a problem with τ-p interpolation due to the presence of noise.
Thus, a method of first arrival picking of seismic refraction data solving the aforementioned problems is desired.